1. Field of the Invention
The present invention relates to tunable filters that that include elastic wave resonators including a LiNbO3 substrate.
2. Description of the Related Art
For example, Japanese Unexamined Patent Application Publication No. 2009-130831 discloses a tunable filter including a serial arm resonator configured of an elastic wave resonator and a parallel arm resonator configured of an elastic wave resonator. In this tunable filter, variable capacitors are connected to the serial arm resonator in series and parallel, respectively. Similarly, variable capacitors are also connected to the parallel arm resonator in series and parallel, respectively.
In a surface acoustic wave resonator used in a tunable filter disclosed in Band Pass Type of Tunable Filters Composed of Ultra Wide Band SAW Resonators By Adjusting Capacitors connected SAW Resonators, 2011 IEEE International Ultrasonics Symposium Proceedings, a Cu electrode is embedded in a groove provided on a surface of a piezoelectric substrate. This surface acoustic wave resonator makes use of resonant characteristics provided by Love waves.
However, the tunable filter disclosed in Japanese Unexamined Patent Application Publication No. 2009-130831 has a problem that its frequency variable width is narrow.
Meanwhile, in the tunable filter disclosed in Band Pass Type of Tunable Filters Composed of Ultra Wide Band SAW Resonators By Adjusting Capacitors Connected SAW Resonators, 2011 IEEE International Ultrasonics Symposium Proceedings, a Cu electrode is embedded in a groove provided on the surface of the piezoelectric substrate used in the tunable filter. This causes a problem that the manufacturing process becomes complicated and costs are increased. In addition, a reflection coefficient per electrode finger is small; note that electrode fingers configure an interdigital transducer (IDT) electrode, a reflector, and so on. This makes a stop-band width narrow and consequently makes it difficult to obtain favorable resonant characteristics in a wide frequency region. Further, the filter characteristic is likely to deteriorate in the vicinity of an anti-resonant frequency.